1. Field of the Invention
The present invention relates to a method for producing a piezoelectric composite substrate having a single-crystal thin film made of a piezoelectric material.
2. Description of the Related Art
In recent years, piezoelectric elements including a piezoelectric composite substrate with a single-crystal thin film made of a piezoelectric material have been developed. The single-crystal thin film made of a piezoelectric material has been produced by a first production process including depositing a piezoelectric material, such as AlN or ZnO, using sputtering or CVD (for example, see Y. Osugi et al.; “Single Crystal FBAR With LiNbO3and LiTaO3”, 2007IEEE MTT-S International Microwave Symposium, pp. 873-876) or a second production process including polishing a single-crystal base made of a piezoelectric material (for example, see M. Bruel; “Silicon On Insulator Material Technology”, Electronics Letters, vol. 31, Issue 14, Jul. 6th, 1995, p. 1201).
Alternatively, a third production process has been used. The third production process includes implanting helium ions into a Z-cut LiNbO3 substrate to form a damaged crystalline layer and removing the damaged crystalline layer by wet etching to produce a piezoelectric single-crystal thin film (see “Fabrication Of Single-Crystal Lithium Niobate Films By Crystal Ion Slicing”, Applied Physics Letters Volume 73, Number 16).
In the first production process, a deposition temperature and deposition conditions for aligning the orientation direction of the crystal axis of a piezoelectric material severely limit the materials which can be used. Even when an alignment film is provided, the film has a c-axis orientation in which the orientation direction of the crystal axis is aligned vertically with respect to a substrate on which piezoelectric film is disposed. In this case, the orientation direction of the crystal axis cannot be controlled. Thus, the mode of vibration cannot be controlled by slanting the orientation direction of the crystal axis.
In the second production process, since the single-crystal base made of the piezoelectric material is polished, most of the piezoelectric material is discarded as shavings, which results in poor efficiency of utilization of the piezoelectric material. Furthermore, the thickness of the resulting thin film varies due to the unevenness in polishing speed and the degree of undulation of the base. Thus, it is difficult to control the polishing so as to provide a uniform thickness, which causes poor productivity.
In the third production process, the use of wet etching increases the production cost. Furthermore, an electrode cannot be provided at a location that is to be wet etched. Thus, when the electrode is arranged on a laminating surface between the piezoelectric single-crystal thin film and a support substrate, the electrode is formed after wet etching. In this case, the thin film has a low stiffness and thus is difficult to handle. Therefore, the thin film is likely to be broken.